As the health-care law expands Medicaid to cover millions more Americans, a new Harvard University study finds that enrollment in the public program significantly increases enrollees’ use of emergency departments.

The research, published Thursday in the journal Science, showed a 40 percent increase in emergency department visits among those low-income adults in Oregon who gained Medicaid coverage in 2008 through a state lottery. This runs counter to some health-care law supporters’ hope that Medicaid coverage would decrease this type of costly medical care, by making it easier for low income adults to see primary care providers.

“I would view it as part of a broader set of evidence that covering people with health insurance doesn’t save money,” says Jonathan Gruber, a health economist at the Massachusetts Institute of Technology, who has also studied Oregon’s Medicaid expansion but is not affiliated with this study. “That was sometimes a misleading motivator for the Affordable Care Act (ACA). The law isn’t designed to save money. It’s designed to improve health, and that’s going to cost money.”

The Science study is part of a much larger research project that looks at Oregon’s 2008 Medicaid expansion, where the state had limited funding to expand the public program to a set number of low-income residents. Oregon used a lottery to determine who would gain access to coverage, and that created a unique opportunity for researchers: The chance to study what happens when some people are randomly assigned to receive health insurance, and others are not.

Previous research on the Oregon Medicaid expansion has found that enrolling in the public program increased hospital visits, primary care trips and prescription drug use. That left an unanswered question: Were new Medicaid enrollees going their primary care doctor instead of the emergency department, or, were they using more of all types of health-care services?

This study suggests the latter answer: With financial barriers removed, Medicaid patients see their primary care doctor more — and also go to the emergency department at an increased frequency. Medicaid enrollees made, on average, 1.43 trips to the emergency department during the 18-month study period, compared to an average of 1.02 visits among those who entered the Medicaid lottery but did not gain coverage.

Medicaid coverage also increased the probability of having any visit to the emergency department by 7 percent. The researchers also looked at the types of visits and found no decline in use of the emergency department for primary care treatable conditions among those who had enrolled in Medicaid coverage.

“Part of what makes emergency department use interesting is there are different theories about what to expect,” lead study author Sarah Taubman, also at the Harvard School of Public Health. “There’s one theory that it increases, because insurance pays for emergency room care that would lead people to use more than if they faced the full cost. The other theory is that, by paying for primary care visits, insurance may lead to a decrease in emergency department use. We looked at this and, taken altogether, we see a net increase.

Twenty-five states expanded their Medicaid programs under the ACA, extending coverage to all adults who earn less than 133 percent of the federal poverty line, about $15,000 for an individual and $31,400 for a family of four. Two more states, Indiana and Pennsylvania, are seeking to move forward with the health-care law program at a later date.

Although the ACA initially mandated that all states expand their Medicaid programs, the Supreme Court ruling in June 2012 found that provision to be too restrictive, allowing each state to decide whether to participate.

Some governors have cited a reduction in emergency department visits as a reason to expand the public program.

“Today, uninsured citizens often turn to emergency rooms for non-urgent care because they don’t have access to primary care doctors — leading to crowded emergency rooms, longer wait times and higher costs,” one fact sheet from Michigan Gov. Rick Snyder, R, states. “By expanding Medicaid, those without insurance will have access to primary care, lowering costs and improving overall health.”

And, when Congress was debating health-care reform in 2009, Health and Human Services Secretary Kathleen Sebelius cited the high number of uninsured Americans being seen at the emergency department as a reason to pass the ACA.

“Our health care system has forced too many uninsured Americans to depend on the emergency room for the care they need,” she said in a July 2009 statement. “We cannot wait for reform that gives all Americans the high-quality, affordable care they need and helps prevent illnesses from turning into emergencies.”

Gruber, the MIT economist, doesn’t see the Harvard study as a compelling case against expanding Medicaid. There are still other benefits to insurance coverage, he says, that aren’t about saving public funding. Separate research on the Oregon expansion, published last spring in the New England Journal of Medicine, found Medicaid enrollees to have significantly lower rates of depression and were more able to pay their medical bills.

“The overall notion is we’re getting people more health care,” Gruber says. “There are huge improvements in mental health. For those who want to argue that expanding Medicaid is a free lunch, this is bad. But that was never the right argument.”